Amide bond synthesis via silver(I) N-heterocyclic carbene-catalyzed and tert-butyl hydroperoxide-mediated oxidative coupling of alcohols with amines under base free conditions

Article abstract of DOI:10.1016/j.tetlet.2019.02.024

We present a base free method for amide bond construction via oxidative coupling of alcohols with amines catalyzed by Silver(I) N-heterocyclic carbenes (Ag(I)-NHCs) and mediated by tert-butyl hydroperoxide (TBHP) in ethanol. The results of controlled experiments suggest that the oxidative coupling proceeds through the formation of aldehyde, then subsequent attack by amine to give hemiaminal, which can then be oxidized to amide.

Full text of DOI:10.1016/j.tetlet.2019.02.024

Accepted Manuscript
Amide bond synthesis via silver(I) N-heterocyclic carbene-catalyzed and tert-
butyl hydroperoxide-mediated oxidative coupling of alcohols with amines under
base free conditions
Ramesh Balaboina, Narasimha Swamy Thirukovela, Ravinder Vadde, Chandra
Sekhar Vasam
PII:
S0040-4039(19)30145-5
https://doi.org/10.1016/j.tetlet.2019.02.024
TETL 50618
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
21 December 2018
9 February 2019
12 February 2019
Please cite this article as: Balaboina, R., Swamy Thirukovela, N., Vadde, R., Sekhar Vasam, C., Amide bond
synthesis via silver(I) N-heterocyclic carbene-catalyzed and tert-butyl hydroperoxide-mediated oxidative coupling
of alcohols with amines under base free conditions, Tetrahedron Letters (2019), doi: https://doi.org/10.1016/j.tetlet.
2019.02.024
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Graphical Abstract
Amide bond synthesis via silver(I) N-heterocyclic
carbene-catalyzed and tert-butyl hydroperoxide
-mediated oxidative coupling of alcohols with amines
under base free conditions
Leave this area blank for abstract info.
Ramesh Balaboina,a Narasimha Swamy
Thirukovela,a Ravinder Vaddea,* and Chandra
Sekhar Vasamb,*

1
Tetrahedron Letters
Amide bond synthesis via silver(I) N-heterocyclic carbene-catalyzed and tert-
butyl hydroperoxide-mediated oxidative coupling of alcohols with amines under
base free conditions
Ramesh Balaboina,a Narasimha Swamy Thirukovela,a Ravinder Vaddea,* and Chandra Sekhar Vasamb,*
aDepartment of Chemistry, Kakatiya University, Warangal, Telangana State-506009, India, Email: ravichemkuc@gmail.com;
bDepartment of Pharmaceutical Chemistry, Telangana University, Nizamabad, Telangana State, India, E-mail: csvasam@telanganauniversity.ac.in
ARTICLE INFO
ABSTRACT
We present a base free method for amide bond construction via oxidative coupling of alcohols
with amines catalyzed by Silver(I) N-heterocyclic carbenes (Ag(I)-NHCs) and mediated by tert-
butyl hydroperoxide(TBHP) in ethanol. The results of controlled experiments suggest that the
oxidative coupling proceeds through the formation of aldehyde, then subsequent attack by amine
to give hemiaminal, which can then be oxidized to amide.
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved.
oxidative amidation
silver(I) N-heterocyclic carbenes
hemiaminal
TBHP
Amide bonds are essential disconnections in organic
retrosynthesis.1 Early methods to form the amide bond using
carboxylic acids or it’s derivatives are super-stoichiometric and
create considerable amount of waste.2 Thus, a variety of catalytic
amide bond formations have been investigated using a range of
organic functionalities as starting materials including alcohols,3
carbonyls,4 oximes,5 primary amides,6 terminal alkynes,7 alkyl
arenes,8 esters,9 aryl boronic acids,10 and aryl halides.11(Figure 1)
Because the catalytic dehydrogenative amidation of alcohol
occurs in the absence of oxidant, it is critical to direct the
transformation of hemiaminal intermediate that formed from
alcohol-amine coupling to the amide or imine.3d,12a Conversely,
catalytic oxidative amidation is generally a tandem process, in
which an oxidant converts selectively the alcohol to aldehyde and
then hemiaminal intermediate selectively to the amide.3k,3m,13
Although the platinum group metal catalyzed amide bond
processes have been established deeper insights of mechanistic
path,3a-d,12a-d however their high cost and toxicity provides
opportunity to explore relatively cheaper and non-toxic transition
metal catalysts. In this context, there are few reports disclosed the
utilization of homogeneous/heterogeneous iron,14 copper,15
silver,16 and zinc17 catalysts in oxidative amidation of alcohol,
which however requires a base or an additive in stoichiometric
amounts or relatively higher catalyst loadings. The report by
Beller17 is the exception to disclose a base free oxidative
amidation via alcohol-amine coupling catalyzed by Zinc/TBHP
system. However, there was no information about the possible
reaction mechanism for the optimized catalytic conditions and
the reported amide yields obtained from aryl and aralkyl amines
are not appreciable.
Figure 1. Catalytic amide bond syntheses.
Since alcohols are relatively abundant and stable starting
materials, catalytic coupling of alcohols with amines via either
dehydrogenative3g-j,12 or oxidative path3k-l,13 has been recognized
as environmentally benign and atom-economic process.

2
Tetrahedron Letters
Silver (I) N-heterocyclic carbenes (Ag(I)-NHCs) are one of the
in ethanol at 60 °C. It was noticed that the oxidative coupling
reaction mediated by O2 gave trace amounts of aldehyde and
imine (Table 1, entry 17), whereas the H2O2 mediated oxidative
coupling gave exclusively amide 3a (Table 1, entry 16).
highly resourceful organometallics since they are (i) carbene
transfer agents to synthesize many other powerful metal-NHC
catalysts, (ii) good antibacterial agents, (iii) precursors for
nanoparticles and liquid crystals and (iv) able to catalyze a
variety of C-C and C-heteroatom bond forming reactions as
emphasized by us18 and others.19 A report by Jiang and
colleagues described the use of Ag(I)-NHCs (NHC = N,N'-
Disubstituted imidazol-2-ylidene based carbenes) in a one-pot
aerobic oxidative coupling of alcohols with amines in toluene to
produce imines in the presence of a base.19a Imines are also an
important class of nitrogen compounds in chemistry.
Figure 2. Ag(I)-NHCs used in the present study
Solvent effect studies were conducted by replacing ethanol with
various non-polar, polar aprotic and polar protic and found that
ethanol solvent is a good choice for the establishment of present
oxidative amidation protocol (see, ESI).
Herein for the first-time we describe the results of a base-
free and additive free, Ag(I)-NHC catalyzed alcohol-amine
oxidative coupling using tert-butyl hydroperoxide (TBHP) as an
oxidant to produce selectively the amide-bond in high yields in a
green solvent ethanol. We also made efforts to derive appropriate
mechanism with the support of controlled experiments.
Table 1. Optimization of catalytic conditions.
Our investigation began with the oxidative amidation of
benzyl alcohol 1a with aniline 2a in ethanol as a model reaction
to optimize the catalytic condition for the selective synthesis of
amide 3a. The results are present in Table 1. Firstly, a control
experiment was conducted using TBHP as the oxidant in the
absence of any catalyst and base at 60 °C i.e. below the b.p. value
of ethanol. The reaction was almost none even after prolonged
reaction periods as observed initially by TLC. Next, the addition
of 5 mol% of Ag-salts (AgOAc, AgOTf, AgSbF6 and AgBF4)as
catalysts to the reaction mixture in the presence of TBHP has
resulted in the formation of desired amide 3a, but the yield was
observed in the range of 18-29% yield after 24 hrs (Table 1,
entries 2-5). Nevertheless, the results indicate Ag(I) metal centre
is capable of promoting base free oxidative amidation of alcohol
with amine selectively to amide using TBHP.
Entry
1
Catalyst(mol%)
---
Oxidant
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
Time
(hrs)
24
Temp Yield
(oC)
60
(%)b
---
2
AgOAc (5)
24
24
24
24
16
16
16
16
18
16
16
16
16
16
16
16
24
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
RT
18
24
29
27
81
81
81
81
65
83
78
82
81
84
64
---
----
3
AgOTf (5)
4
AgSbF6(5)
5
AgBF4(5)
6
Ag(I)-NHC (a) (5)
Ag(I)-NHC (a) (4)
Ag(I)-NHC (a) (3)
Ag(I)-NHC (a) (2)
Ag(I)-NHC (a) (1)
Ag(I)-NHC (b) (2)
Ag(I)-NHC (c) (2)
Ag(I)-NHC (d) (2)
Ag(I)-NHC (e) (2)
Ag(I)-NHC (f) (2)
Ag(I)-NHC (a) (2)
Ag(I)-NHC (a) (2)
Ag(I)-NHC (a) (2)
7
In order to improve the catalytic conditions (yield and reaction
times), we next investigated the efficacy of pre-synthesized
Ag(I)-NHCs (Figure 2, a-f) as catalysts in the above oxidative
coupling. At first, the catalytic ability of 5 mol% Ag(I)-NHC (R
= (a) mesityl, Figure 2) was tested in the coupling of 1a with 2a
and obtained corresponding amide 3a in 81% yield after 16 hours
(Table 1, entry 6). Since Ag(I)-NHCs are organometallic
catalysts, we have also carried out additional experiments using
4, 3 and 2 mol% concentration of Ag(I)-NHCs in the oxidative
coupling 1a with 2a. To our surprise, there was no change in the
yield of 3a (Table 1, entries 6-9). However, there was a sudden
decrease in the yield of amide 3a when 1 mol% of Ag(I)-NHC
(a) was utilized. (Table 1, entry 10). The other 2 mol% Ag(I)-
NHCs depicted in Figure 2 have also performed well as catalysts
in the oxidative coupling of 1a with 2a in providing high yields
of amide 3a (Table 1,entries 11-15) and indicates that the Ag(I)-
NHCs used in our work are more efficient than normal Ag(I)-salt
catalysts in ethanol. On the other hand, oxidative coupling
between 1a and 2a at room temperature was unsuccessful. Only
the TBHP-promoted benzyl alcohol oxidation to benzaldehyde
was noticed (Table 1, entry 18).
8
9
10
11
12
13
14
15
16
17
18
air
TBHP
a Reaction conditions: Reaction was carried out using 1a (1 mmol), 2a (1.2
mmol) and TBHP (3 mmol) in ethanol (3 mL). b Isolated yields after column
chromatography.
Having the optimized conditions in hand (alcohol/amine = 1/1.2
ratio, 3 equivalents of TBHP, 2 mol% Ag(I)-NHC, 60 °C,
ethanol) we have further extended this protocol to synthesize
different primary and secondary amides and the results were
The catalytic efficiency of Ag(I)-NHC (a) was also studied in the
presence of H2O2 and O2 promoted coupling between 1a and 2a

3
shown in table 2. The substituted benzyl alcohols containing
electron-withdrawing groups transformed easily to corresponding
primary amides (Table 2, entries 3e and 3i-3l) than those
containing electron-donating groups (Table 2, entries 3f-3h).
With respect to the reactivity of amines, it found that all the
linear aryl primary amines have coupled efficiently with the
alcohol partners in providing respective primary amides (Table 2,
3i-3q). In the case of anilines, those anilines bearing electron-
releasing groups have given slightly higher yields of primary
amides (Table 2, entries 3c-3d) than the other anilines (Table 2,
entries 3g-3h).
(iii) When benzaldehyde 1aa was coupled with aniline 2a
in the presence of TBHP, formation amide 3a was noticed.
From the equation i, ii, and iii it is confirmed that our
approach proceeds through the in situ formation of aldehydes
rather than ester and TBHP oxidant is responsible for the
formation of desired amides through the oxidation of in situ
generated hemiaminal that obtained by the coupling
benzaldehyde with aniline in ethanol.
Besides, we have also utilized secondary amines as partners to
alcohols in oxidative coupling. It was noticed that the oxidative
coupling reactions using both the acyclic and aryl alkyl
secondary amines (Table 2, entries 3r-3v) are less efficient than
cyclic secondary amines (Table 2, entries 3w-3y). All the
1
synthesized products were characterized by H NMR, 13C NMR
and mass spectrometry.
Table 2. Substrate scope.
Scheme 1. Controlled experiments.
Based on the above supporting studies and from the literature
support14b,14c,19a,20 we proposed the plausible mechanism as shown
in scheme 2. Initially Alkoxide-silver NHC-complex
intermediate A was formed by the coordination of NHC-Ag-Cl to
the alcohol. This intermediate A is activated by TBHP to afford
the intermediate B in which Ag-metal centre having (+2)
oxidation state. Later, intermolecular β-hydrogen abstraction of
intermediate B takes place to provide the aldehyde and tert-
butanol. Finally, the catalytic cycle is completed by the attack of
labile Cl¯ ion on free radical NHC-Ag-O· to regenerate NHC-Ag-
Cl along with the release of molecular oxygen. The regenerated
NHC-Ag-Cl will start the next phase of catalytic cycle by
coordinating to in situ formed aldehyde which subsequently
reacts with amine to form hemiaminal intermediate C. The
hemiaminal C reacts with TBHP to give the intermediate D.
Then the same process was observed as that of intermediate B to
give desired amides and tBuOH (intermediate D to amide).
Finally, the NHC-Ag-Cl is renewed by the reaction of labile Cl¯
ion on NHC-Ag-O· radical by releasing molecular oxygen.
a Reaction conditions: 1(a-j) (1 mmol), 2(a-m) (1.2mmol), Ag(I)-NHC (a) (2
mol%) and TBHP (3 mmol) in ethanol (3 mL) at 60 oC temperature for 12-18
hours. b Isolated yields after column chromatography.
In order to understand the reaction mechanism, we have done few
supporting studies (Scheme 1).
(i) When the benzyl alcohol 1a was treated only with TBHP
in the presence of Ag(I)-NHC, exclusively benzaldehyde 1aa was
formed in 91 % yield (CC) in ethanol at 60 °C.
Scheme 2. Proposed mechanism.
(ii) When benzaldehyde 1aa was coupled with aniline 2a in
the absence of TBHP, formation of schiff base 1ab was noticed.
In conclusion, we have developed a base free direct
amidation from alcohols and primary/secondary amines through
a tandem oxidation process that catalyzed by Ag(I)-NHC

4
Tetrahedron Letters
Mentzel, U. V.; Jensen, T.; Fristrup, P.; Riisager, A. Chem.
Commun. 2012, 48, 2427-2429.
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catalytic conditions developed in the present work for the amide
bond construction are comparable or relatively better than the
previous works in terms of yield and selectivity.
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Acknowledgements
The authors are thankful to the DST-SERB, New Delhi, India
File No. SR/S1/IC-31/2011, DST-India (DST/INT/SA/P-15/2011
Indo-South Africa project) for financial support.
Supplementary data
Experimental details, characterization data of 3(a-y) products can
be found, in the online version, at http://dx.doi.org/
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6
Tetrahedron Letters
Highlights of this manuscript are;
 Ag(I)-NHC catalysed base-free synthesis of
amide-bond
coupling.
from
alcohol-amine
 TBHP is an effective oxidant than H2O2 &
O2 to produce selectively amide than
imine.
 One-pot double-oxidation of alcohol to
amide bond proceeds via hemiaminal
intermediate.
 Optimized conditions can also limit the
formation of related byproducts of
alcohols.